1. Field of the Invention
The present invention relates to movable contact units for panel switches that typically configure control panels of electronic devices, and panel switches configured using the movable contact unit.
2. Background Art
Mobile electronic devices such as cellular phones and PDAs (Personal Digital/Data Assistants) are increasingly being used in recent years. These electronic devices are being constantly downsized to improve their portability, so the panel switches used in their control panels need to be made smaller. A conventional movable contact unit employed in these panel switches and a panel switch configured with a conventional movable contact unit are described next with reference to FIGS. 4 to 6. FIG. 4 is a sectional view of the conventional movable contact unit, FIG. 5 is an exploded perspective view of the same movable contact unit, and FIG. 6 is a sectional view of a panel switch configured using the same movable contact unit.
In FIGS. 4 and 5, the movable contact unit includes movable contact 1 and base sheet 2. Movable contact 1 consists of a thin resilient metal sheet that is dome-shaped, with convex top and open bottom, and its circumference is round. Base sheet 2 is an insulating film, made typically of polyethylene terephthalate (PET). Base sheet 2 adheres to and supports dome tip 1A of each of movable contacts 1 disposed at predetermined positions with its undersurface where adhesive 3 is applied.
Adhesive 3 on the undersurface of base sheet 2 is applied to the entire undersurface of base sheet 2 except for a portion corresponding to a peripheral area of each movable contact 1. In other words, the portion corresponding to the peripheral area of each movable contact 1 forms an independent circular ring of non-adhesive area 3A. Dome tip 1A is adhered to adhesive 3 applied to a circular area inside non-adhesive area 3A, and thus each movable contact 1 is held by the undersurface of base sheet 2.
As shown in FIG. 6, the conventional movable contact unit as configured above is used, for example, in a panel switch. In FIG. 6, each fixed contact 4 configured with a pair of outer fixed contact 4A and central contact 4B is disposed respectively at positions corresponding to each of movable contacts 1 in the movable contact unit. Peripheral lower end 1B of each movable contact 1 is positioned on each outer fixed contact 4A, and bonded with adhesive 3 on the undersurface of base sheet 2.
In FIG. 6, each outer fixed contact 4A is electrically contiguous with corresponding movable contact 1, and the undersurface of dome tip 1A of each movable contact 1 faces corresponding central fixed contact 4B with a distance in between. Movable contacts 1 disposed and adhered at predetermined positions on the undersurface of base sheet 2 respectively configure independent switches, and thus the panel switch is configured as a whole.
Non-adhesive area 3A is provided on the area corresponding to the periphery of each movable contact 1 in the application pattern of adhesive 3 to avoid unwanted adhesive 3 from extending to underneath the lower end of movable contact 1 when configuring the panel switch. A protective sheet made of an insulating film is attached to and covers the entire undersurface of base sheet 2 of the movable contact unit to protect movable contact 1 from dust, etc., during transportation and storage. This protective sheet is usually removed when bonding the movable contact unit onto wiring board 5.
Each switch on the panel switch operates such that dome tip 1A of movable contact 1 is depressed via base sheet 2 using the push button (not illustrated) of the electronic device. This pressing force resiliently deforms the dome portion of movable contact 1, giving tactile feedback, and the undersurface of dome tip 1A contacts central fixed contact 4B underneath. Outer fixed contact 4A and central fixed contact 4B thus becomes electrically contiguous, at which point the switch is turned on.
When the pushing force is released, the dome portion of movable contact 1 reverts to its original dome shape with convex tip at the top, giving tactile feedback, and thus the undersurface of dome tip 1A separates from central fixed point 4B. This breaks the electrical contact between outer fixed contact 4A and central fixed contact 4B as shown in FIG. 4, and thus the switch returns to the original off state. These conventional configurations of the movable contact unit and panel switch are disclosed, for example, in the Japanese Patent Unexamined Publication No. H10-188728.
The above conventional movable contact unit and the panel switch using the conventional movable contact unit have been downsized by narrowing the distance between the edge of base sheet 2 and movable contact 1 disposed near the edge, in line with downsizing of devices where the switch is installed.
However, it is necessary to ensure that base sheet 2 does not peel off from the edge when the movable contact unit is bonded to wiring board 5 and the panel switch is configured, including the times when each switch is operated. Width A shown in FIG. 6 where adhesive 3 is applied between the edge of base sheet 2 and circular-ring non-adhesive area 3A needs to be set to a distance that can safely retain an adhesive strength state. Accordingly, the above way of downsizing the conventional movable contact unit and panel switch is simple and effective but has its limits.